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In the absence of global plate tectonics, mantle convection and plume–lithosphere interaction are the main drivers of surface deformation on Venus. Among documented tectonic structures, circular volcano-tectonic features known as coronae may be the clearest surface manifestations of mantle plumes and hold clues to the global Venusian tectonic regime. Yet, the exact processes underlying coronae formation and the reasons for their diverse morphologies remain controversial. Here we use three-dimensional thermomechanical numerical simulations of impingement of a thermal mantle plume on the Venusian lithosphere to assess the origin and diversity of large Venusian coronae. The ability of the mantle plume to penetrate into the Venusian lithosphere results in four main outcomes: lithospheric dripping, short-lived subduction, embedded plume and plume underplating. During the first three scenarios, plume penetration and spreading induce crustal thickness variations that eventually lead to a final topographic isostasy-driven topographic inversion from circular trenches surrounding elevated interiors to raised rims surrounding inner depressions, as observed on many Venusian coronae. Different corona structures may represent not only different styles of plume–lithosphere interactions but also different stages in evolution. A morphological analysis of large existing coronae leads to the conclusion that at least 37 large coronae (including the largest Artemis corona) are active, providing evidence for widespread ongoing plume activity on Venus. Thermomechanical modelling shows that the formation and diverse morphologies of coronae on Venus can be explained by interactions between the lithosphere and impinging mantle plumes. Some corona structures are consistent with ongoing plume activity.

International audience; Our planet is in crisis! The latest report of the United Nations Intergovernmental Panel on Climate Change (IPCC AR6) confirms that human influence is causing widespread, rapid, and intensifying changes in our weather and climate that are affecting every region on Earth in multiple ways. With every additional ton of carbon we emit, the frequency and intensity of storms, floods, droughts, and fires become greater and the effects on the environment and on human health and civilization become more severe.As geoscientists and journal editors, most of us have been accustomed to being on the leading edge of human knowledge and understanding of climate change, where we deal in objectivity, uncertainty, and debate, but now we find ourselves at the core of this climate crisis. It is no longer scientific discoveries at stake, but also humanity itself. This is an uncomfortable place for many of us. We are trained to be dispassionate observers and cautious thinkers, yet the alarming rate of recent climate change impels us to turn our attention directly toward mitigating this impending crisis.We are making a plea for collective action: we must make the switch to a green economy, put a just and effective price on carbon now, and consider a portfolio of other equitable public investments in climate solutions. These actions will ensure that the true costs and risks of burning fossil fuels are accounted for and global carbon emissions are rapidly reduced. Rich countries must lead the way in making drastic cuts to carbon emissions and in helping low- and middle-income countries to develop sustainably. We are running out of time.For decades, American Geophysical Union (AGU) journals have been at the forefront of documenting human-caused climate change and warning of a worsening climate crisis. Over 2,000 publications from AGU journals are cited in the new IPCC AR6 report. But we too can do more than just document and scientifically explain the ongoing crisis—our profession must help lead the way to solutions.Finding solutions and adapting to change have become not only necessary, but essential in ensuring safe, sustainable, and healthy human communities in the future. The geosciences have an essential role to play in these efforts by pivoting toward more cross-sector, solution-based science. To help lead this vision, the AGU is adding a new publication forum for community science in partnership with associations outside the geosciences. This forum will enhance interactions among AGU's existing, more disciplinary journals and give local communities a voice in leading solutions to global challenges.We are scientists, but we also have families and loved ones alongside our fellow citizens on this planet. The time to bridge the divide between scientist and citizen, head and heart, is now. The lead-up to the 2021 UN Climate Change Conference, COP26, being held in Glasgow in November, is our “last best chance” to urge world leaders to come together and commit to keeping climate change and its devastating impacts in check.

SUMMARY Inversions of planetary gravity are aimed at constraining the mass distribution within a planet or moon. In many cases, constraints on the interior structure of the planet, such as the depth of density anomalies, must be assumed a priori, to reduce the non-uniqueness inherent in gravity inversions. Here, we propose an alternative approach that embraces the non-uniqueness of gravity inversions and provides a more complete view of related uncertainties. We developed a Transdimensional Hierarchical Bayesian (THB) inversion algorithm that provides an ensemble of mass distribution models compatible with the gravitational field of the body. Using this ensemble of models instead of only one, it is possible to quantify the range of interior parameters that produce a good fit to the gravity acceleration data. To represent the interior structure of the planet or moon, we parametrize mass excess or deficits with point masses. We test this method with synthetic data and, in each test, the algorithm is able to find models that fit the gravity data of the body very well. Three of the target or test models used contain only point mass anomalies. When all the point mass anomalies in the target model produce gravity anomalies of similar magnitudes and the signals from each anomaly are well separated, the algorithm recovers the correct location, number and magnitude of the point mass anomalies. When the gravity acceleration data of a model is produced mostly by a subset of the point mass anomalies in the target model, the algorithm only recovers the dominant anomalies. The fourth target model is composed of spherical caps representing lunar mass concentration (mascons) under major impact basins. The algorithm finds the correct location of the centre of the mascons but fails to find their correct outline or shape. Although the inversion results appear less sharp than the ones obtained by classical inversion methods, our THB algorithm provides an objective way to analyse the interior of planetary bodies that includes epistemic uncertainty.

International audience; Peer reviewing is the selfless act of evaluating and providing feedback on scientific papers submitted in journals like JGR: Planets. Every review is written with the objective of clarifying the article and often contains suggestions for additional analyses or topics of discussion. Reviewers often spend hours examining the rigor of the methods, the validity of the results, and the significance of the conclusions in the submitted manuscripts. The extra effort of our pool of reviewers in a year when we all needed to adjust our daily routines in response to a pandemic has not gone unnoticed. In 2020, JGR: Planets benefited from 1,168 reviews provided by 705 unique reviewers. We are particularly grateful to the scientists who remained involved in the review process. We also understand that many of us had to reduce our reviewing commitment this year. Hopefully, the coming year will see a return to normalcy in your lives. To our reviewers, past, present, and future, the entire editorial board of JGR: Planets wishes to express here our most heartfelt gratitude. Thank you!

Venus Corona and Tessera Explorer (VeCaTEx) would use an aerobot to descend repeatedly beneath the dense clouds for imaging targeted area of the surface in the near infrared to address six of the prime investigations prioritized by VEXAG. The technologies needed could be matured during the next decade.

International audience; Science is at its best when shared widely both within the scientific community and to the broader public. To maintain trust in science while simultaneously maximizing openness, it is important that every paper is carefully vetted. For this we rely on many volunteers, especially our peer reviewers. Their work is key for maintaining the integrity of scientific journals and to ensure the highest standards of scientific rigor. Reviewers verify that the assumptions, the methods, and the inferences made in our papers are sound. Many spend hours going over derivations, analyses, and evaluating the novelty and significance of the submitted manuscript. In 2019, JGR Planets benefited from 901 reviews provided by 584 unique reviewers. On behalf of the entire editorial board of JGR Planets and the entire planetary science community, we want to express here our most heartfelt gratitude to all of you. Thank you!

As the Earth's primary mode of planetary cooling, the oceanic plate is created at mid-ocean ridges, transported across the planet's surface, and destroyed at subduction zones. The evolution of its buoyancy and rheology during its lifespan maintains the coherence of the plate as a distinct geological entity and controls the localised deformation and vertical material exchange at plate boundaries, which enables the horizontal ocean-plate movements. These motions intimately link the oceanic plate to the overarching overturn of Earth's mantle: The plate forms out of rising mantle material at spreading ridges; it cools the Earth's interior as the cold thermal boundary layer to mantle convection; and its sinking portions drive not only the plate itself but also dominate global flow in the mantle. We scrutinise here the entire life cycle of the oceanic plate, starting with its birth at the mid-ocean ridge, including the thermal, rheological, and chemical conditions of initiation, followed by plate maturation as it ages and cools while crossing the seafloor, and finishing with the dynamics of plate destruction as it retires at the subduction zone to become a deeper part of Earth's convective system. We find that the full range of dynamic behaviour of the oceanic plate, including its forcing and overall framework within Earth's convecting system, is not fully captured by the existing concept of Plate Tectonics, which describes solely the horizontal surface kinematics of all plates. Therefore, we introduce a more specific and at the same time more integral concept named “Ocean-Plate Tectonics” that more specifically describes the dynamic life of the oceanic plate and accounts for the knowledge gained during the past 50 years. This “Ocean-Plate Tectonics” must have emerged on Earth at least 1 Billion years ago, and dominates Earth's dynamics today.

MeltMigrator is a MATLAB®-based melt migration software developed to process three-dimensional mantle temperature and velocity data from user-supplied numerical models of mid-ocean ridges, calculate melt production and melt migration trajectories in the mantle, estimate melt flux along plate boundaries, and predict crustal thickness distribution on the seafloor. MeltMigrator is also capable of calculating compositional evolution depending on the choice of petrologic melting model. Programmed in modules, MeltMigrator is highly customizable and can be expanded to a wide range of applications. We have applied it to complex mid-ocean ridge model settings, including transform faults, oblique segments, ridge migration, asymmetrical spreading, background mantle flow, and ridge-plume interaction. In this technical report, we include an example application to a segmented mid-ocean ridge. MeltMigrator is available as a supplement to this paper, and it is also available from GitHub and the University of Maryland Geodynamics Group website.

Subduction dynamics can be understood as the result of the balance between driving and resisting forces. Previous work has traditionally regarded gravitational slab pull and viscous mantle drag as the main driving and resistive forces for plate motion respectively. However, this paradigm fails to explain many of the observations in subduction zones. For example, subducting plate velocity varies significantly along-strike in many subduction zones and this variation is not correlated to the age of subducting lithosphere. Here we present three-dimensional and time-dependent numerical models of subduction. We show that along-strike variations of the overriding plate thermal structure can lead to along-strike variations in subducting plate velocity. In turn, velocity variations lead to significant migration of the Euler pole over time. Our results show that the subducting plate is slower beneath the colder portion of the overriding plate due to two related mechanisms. First, the mantle wedge beneath the colder portion of the overriding plate is more viscous, which increases mantle drag. Second, where the mantle wedge is more viscous, hydrodynamic suction increases, leading to a lower slab dip. Both factors contribute to decreasing subducting plate velocity in the region; therefore, if the overriding plate is not uniform, the resulting velocity varies significantly along-strike, which causes the Euler pole to migrate closer to the subducting plate. We present a new mechanism to explain observations of subducting plate velocity in the Cocos and Nazca plates. These results shed new light on the balance of forces that control subduction dynamics and prove that future studies should take into consideration the three-dimensional structure of the overriding plate., This work was supported by Spanish Ministry of Economy and Competitiveness projects CGL2012-37222 and CGL2014-58821-C2-1-R. M.I. Billen acknowledges support from NSF grant 1246864. L.G.J. Montesi acknowledges support from NSF grant OCE 10-60878.

Observations of dunite channels in ophiolites and uranium-series disequilibria in mid-ocean ridge basalt suggest that melt transport in the upper mantle beneath mid-ocean ridges is strongly channelized. We present experimental evidence that spatial variations in mineralogy can also focus melt on the grain-scale. This lithologic melt partitioning, which results from differences in the interfacial energies associated with olivine-melt and orthopyroxene-melt boundaries, may complement other melt focusing mechanisms in the upper mantle such as mechanical shear and pyroxene dissolution. We document here lithologic melt partitioning in olivine/orthopyroxene-basaltic melt samples containing nominal olivine to orthopyroxene ratio of 3 to 2 and melt fractions of 0.02 to 0.20. Experimental samples were imaged using synchrotron-based X-ray micro-computed tomography at a resolution of 700 nm per voxel. By analyzing the local melt fraction distributions associated with olivine and orthopyroxene (opx) grains in each sample, we found that the melt partitioning coefficient, i.e., the ratio of melt fraction around olivine to that around orthopyroxene grains, varies between 1.1 and 1.6. The permeability and electrical conductivity of our digital samples were estimated using numerical models and compared to those of samples containing only olivine and basaltic melt. Our results suggest that lithologic melt partitioning and preferential localization of melt around olivine grains might play a role in melt focusing, potentially enhancing average melt ascent velocities.

Estimates of melt content beneath fast-spreading mid-ocean ridges inferred from magnetotelluric tomography (MT) vary between 0.01 and 0.10. Much of this variation may stem from a lack of understanding of how the grain-scale melt geometry influences the bulk electrical conductivity of a partially molten rock, especially at low melt fraction. We compute bulk electrical conductivity of olivine–basalt aggregates over 0.02 to 0.20 melt fraction by simulating electric current in experimentally obtained partially molten geometries. Olivine–basalt aggregates were synthesized by hot-pressing San Carlos olivine and high-alumina basalt in a solid–medium piston-cylinder apparatus. Run conditions for experimental charges were 1.5 GPa and 1350 °C. Upon completion, charges were quenched and cored. Samples were imaged using synchrotron X-ray micro-computed tomography (μ-CT). The resulting high-resolution, 3-dimensional (3-D) image of the melt distribution constitutes a digital rock sample, on which numerical simulations were conducted to estimate material properties. To compute bulk electrical conductivity, we simulated a direct current measurement by solving the current continuity equation, assuming electrical conductivities for olivine and melt. An application of Ohm's Law yields the bulk electrical conductivity of the partially molten region. The bulk electrical conductivity values for nominally dry materials follow a power-law relationship σ bulk = C σ melt ϕ m with fit parameters m = 1.3 ± 0.3 and C = 0.66 ± 0.06 . Laminar fluid flow simulations were conducted on the same partially molten geometries to obtain permeability, and the respective pathways for electrical current and fluid flow over the same melt geometry were compared. Our results indicate that the pathways for flow fluid are different from those for electric current. Electrical tortuosity is lower than fluid flow tortuosity. The simulation results are compared to existing experimental data, and the potential influence of volatiles and melt films on electrical conductivity of partially molten rocks is discussed.

Global compilations of subslab shear wave splitting parameters show a mix of trench-parallel and trench-perpendicular fast directions that often directly contradict predictions from two-dimensional models of slab-entrained flow. Here we show that subslab anisotropy is consistent with three-dimensional geodynamic models that feature the interaction between subducting slabs and regional mantle flow. Each model represents a specific region for which high-quality source-side shear wave splitting data are available. We compare the distribution of finite strain in the models with shear wave splitting observations, showing that both trench-parallel and trench-perpendicular fast directions can be explained by deflection of regional mantle flow around or beneath subducted slabs. Subslab maximum elongation directions calculated from our models depend on a combination of geometry factors (such as slab dip angle and maximum depth), mechanical parameters (such as decoupling between the slab and the subjacent mantle), and the orientation and magnitude of the regional mantle flow.

Three-dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench-parallel subslab seismic anisotropy. Subslab flow becomes inherently three-dimensional when slab-driven flow interacts with a wide variety of ambient background mantle flow conditions. This interaction depends on slab geometries, mechanical coupling parameters, and lower mantle viscosities. Deflection of subslab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient, background mantle flow. Background mantle flow can become trench-perpendicular or trench-parallel subslab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or toward the edge of the slab. The first case is especially prominent in models with short slabs that do not penetrate into the lower mantle. The second case is especially prominent in models with long, steep slabs. The results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it. High levels of coupling create a boundary layer of trench-perpendicular entrained flow, pushing the deflection due to the obstructing slab away from the slab. We compare our subslab flow model predictions with a global set of seismic anisotropy fast directions in the subslab mantle, and find generally good agreement between the anisotropy observations (dominantly trench-parallel or trench-perpendicular) and the mantle flow directions predicted for decoupled systems.

Subduction-related transport of water into the mantle has significant dynamical and geochemical implications. Dehydration of hydrous phases within the slab can introduce water into the transition zone and lower mantle, potentially hydrating nominally anhydrous minerals (NAM) and impacting the viscosity and density structure of the mantle over a wide area. We present models of fluid transport and mantle hydration in the vicinity of a deeply subducting slab, focusing on the fate of water released by deep dehydration reaction in the subducted serpentinized mantle. A sharp decrease in water storage capacity across the lower boundary of the transition zone may produce “secondary dehydration” of hydrated NAM, leading to precipitation of a hydrous fluid and heterogeneous hydration of the transition zone. Rapid fluid migration relative to the solid flow field can lead to a broad region of diffuse hydration within the upper mantle wedge and the potential for localized melt regions at the top of the transition zone coincident with fluid pathways. Slower fluid migration instead implies that the fluid phase can be transported deep into the lower mantle. Water stabilized in NAM and as a free fluid can initiate upwelling within and above the transition zone. A less abrupt change in water storage capacity across the base of the transition zone leads to high NAM water contents in a channel adjacent to the slab where viscosity is reduced. However, seismic and electromagnetic observations of hydration in the transition zone are most compatible with a sudden drop of water storage capacity.

At the Galapagos triple junction, the Cocos and Nazca plates are broken by a succession of transient rifts north and south of the Cocos–Nazca (C–N) Rift. Modeling has suggested that each rift initiated at the East Pacific Rise (EPR), its location controlled by the distance of the C–N Rift tip from the EPR. Evidence on the Pacific plate confirms that each transient rift formed a true RRR triple junction with the EPR and clarifies the history of the region. At ∼1.5 Ma the triple junctions began jumping rapidly toward the C–N Rift suggesting that the C–N Rift tip moved closer to the EPR. Pacific abyssal hills became broad and shallow indicating enhanced magma supply to the region. At ∼1.4 Ma, the Galapagos microplate developed when extension became fixed on the southern transient rift to form the South scarp of the future Dietz rift basin. Lavas flooded the area and a Galapagos–Nazca magmatic spreading center initiated at the EPR. We suggest that a hotspot was approaching the southern triple junction from the west. The hotspot crossed to the Nazca plate ∼1.25 Ma. Dietz seamount formed within the young spreading center, dikes intruded Dietz rift basin, and eruptions built volcanic ridges. Since ∼0.8 Ma magmatic spreading has jumped northward twice, most recently to Dietz volcanic ridge. Amagmatic extension to the east has formed the large North scarp of Dietz rift basin. Northward jumping of the southern triple junction has maintained the microplate boundary close to the proposed hotspot.

The Rembrandt basin is crosscut by the largest fault scarp on Mercury, Enterprise Rupes, and a second scarp complex, Belgica Rupes, extends to the basin's rim. Topographic data derived from MESSENGER orbital stereo images show that these tectonic landforms bound a broad, relatively flat-floored valley with a mean width of ~400 km. Crosscutting relations suggest that the accumulation of structural relief likely postdates the formation and volcanic infilling of the Rembrandt basin. The valley floor, bound by fault scarps of opposite vergence, is significantly offset below the elevation of the back-scarp terrains. Along with an offset section of Rembrandt's rim, the elevation differences are evidence that the valley floor was lowered as a result of the formation of bounding fault scarps. The localization of the widely spaced thrust faults of Enterprise and Belgica Rupis and the offset of the valley floor may be the result of long-wavelength buckling of Mercury's lithosphere.

SUMMARY Volcanic gaps are segments of subduction zones that lack the volcanic activity usually found at these convergent margins. They are regions where the necessary conditions to produce melt may appear favourable, but where volcanoes are surprisingly absent from the surface. In this study, we present a new model that can explain the occurrence of such volcanic gaps. It is based on seismic imaging and geodynamic modelling of the Denali volcanic gap, a ∼400-km-wide region at the eastern end of the Alaska-Aleutian subduction zone. Here, the thick crust of the Pacific Plate and Yakutat terrane subduct at shallow angle beneath North America. A high-resolution seismic profile clearly images the subducting crust undergoing progressive dehydration between 50 and 120 km depth, and a negative subhorizontal velocity contrast at 60 km depth in the overlying mantle wedge. We interpret this 60 km discontinuity as marking the top of a layer of partial melt that pools at the base of the overriding plate. In steadystate subduction models, melt accumulates at the apex of a vaulted mantle wedge, the ‘pinch zone’, from where it may break through the overlying lithosphere to the surface. Beneath the Denali volcanic gap, the pinch zone is absent (or greatly reduced) because shallow subduction of the Yakutat terrane progressively cools the system, and causes the slab to advance and replace the hot core of the mantle wedge. This regime can be seen as the opposite of subduction rollback. It prevents the formation of a pinch zone, reduces the length of the melting column and causes melt to pool at the base of the overriding plate, thus inhibiting magma generation and extraction.

International audience; Much debate exists concerning the strength distribution of the continental lithosphere, how it controls lithosphere-scale strain localization and hence enables plate tectonics. No rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid-like behaviour of plate interiors. Here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution. Geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle, respectively. The positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remain

The Galapagos triple junction is not a simple ridge-ridge-ridge (RRR) triple junction. The Cocos-Nazca Rift (C-N Rift) tip does not meet the East Pacific Rise (EPR). Instead, two secondary rifts form the link: Incipient Rift at 2°40′N and Dietz Deep volcanic ridge, the southern boundary of the Galapagos microplate (GMP), at 1°10′N. Recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the GMP (∼1.5 Ma). South of C-N Rift a band of northeast-trending cracks cuts EPR-generated abyssal hills. It is a mirror image of a band of cracks previously identified north of C-N Rift on the same age crust. In both areas, the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the EPR spreading center and cut eastward into pre-existing topography. Each crack formed a short-lived triple junction until it was abandoned and a new crack and triple junction initiated nearby. Between 2.5 and 1.5 Ma, the pattern of cracking is remarkably symmetric about C-N Rift providing support for a crack interaction model in which crack initiation at the EPR axis is controlled by stresses associated with the tip of the westward-propagating C-N Rift. The model also shows that offsets of the EPR axis may explain times when cracking is not symmetric. South of C-N Rift, cracks are observed on seafloor as old as 10.5 Ma suggesting that this triple junction has not been a simple RRR triple junction during that time.

[1] Crustal thickness variations at the ultraslow spreading 10–16°E region of the Southwest Indian Ridge are used to constrain melt migration processes. In the study area, ridge morphology correlates with the obliquity of the ridge axis with respect to the spreading direction. A long oblique “supersegment”, nearly devoid of magmatism, is flanked at either end by robust magmatic centers (Joseph Mayes Seamount and Narrowgate segment) of much lesser obliquity. Plate-driven mantle flow and temperature structure are calculated in 3-D based on the observed ridge segmentation. Melt extraction is assumed to occur in three steps: (1) vertical migration out of the melting region, (2) focusing along an inclined permeability barrier, and (3) extraction when the melt enters a region shallower than ∼35 km within 5 km of the ridge axis. No crust is predicted in our model along the oblique supersegment. The formation of Joseph Mayes Seamount is consistent with an on-axis melt anomaly induced by the local orthogonal spreading. The crustal thickness anomaly at Narrowgate results from melt extracted at a tectonic damage zone as it travels along the axis toward regions of lesser obliquity. Orthogonal spreading enhances the Narrowgate crustal thickness anomaly but is not necessary for it. The lack of a residual mantle Bouguer gravity high along the oblique supersegment can be explained by deep serpentization of the upper mantle permissible by the thermal structure of this ridge segment. Buoyancy-driven upwelling and/or mantle heterogeneities are not required to explain the extreme focusing of melt in the study area.

[1] Throughout the global mid-ocean ridge system, transform faults offset spreading centers. Conductive cooling may be more efficient beneath transform faults, producing a thickened lithosphere that directs melt away from the transform. However, recent observations of thickened crust along transform faults at fast ridges suggest melt redistribution toward transforms, intra-crustal melt production, or efficient extraction of melt. We apply a 3-D model of melt migration and extraction along an oceanic transform domain bounded by ridge segments. Melt is assumed to travel vertically before collecting and migrating beneath a low-permeability boundary inclined towards the ridge axis. A melt extraction zone, which may be geologically interpreted as the presence of faults and/or dikes leading to rapid lateral and vertical melt migration toward plate boundaries, affects the pattern of crustal accretion at segmented ridges. First, we examine a generic ridge-transform-ridge geometry and then a model that represents the Siqueiros transform on the East Pacific Rise. On the basis of crustal thickness variations within the intra-transform spreading centers along the fast-slipping Siqueiros fault, we constrain the presence of a melt extraction zone within 10 km of the transform zone.

The permeability of the upper mantle controls melt segregation beneath spreading centers. Reconciling contradictory geochemical and geophysical observations at ocean ridges requires a better understanding of transport properties in partially molten rocks. Using x-ray synchrotron microtomography, we obtained three-dimensional data on melt distribution for mantle peridotite with various melt fractions. At melt fractions as low as 0.02, triple junctions along grain edges dominated the melt network; there was no evidence of an abrupt change in the fundamental character of melt extraction as melt fraction increased to 0.2. The porosity of the partially molten region beneath ocean ridges is therefore controlled by a balance between viscous compaction and melting rate, not by a change in melt topology.

Melt focusing at mid-ocean ridges is necessary to explain the narrowness of the zone of crustal accretion and the formation of large but localized on-axis seamounts at slow and ultraslow spreading centers. It has been proposed that melt focusing is facilitated by the presence of a barrier to upward melt migration at the base of the thermal boundary layer (TBL). We assess the development of a melt impermeable boundary by modeling the geochemical evolution and crystallization history of melts as they rise into the TBL of mid-ocean ridges with different spreading rates. A permeability barrier, associated with a crystallization front controlled by the conductive thermal regime, exists for melt trajectories at slow to fast spreading ridges (≥10 mm/yr half rate). The effective lateral scope of the barrier, where the slope of the barrier exceeds a critical value that allows buoyant melt transport to the axis, generally increases with spreading rate. At all distances from the axis at ultraslow ridges and off-axis at slow spreading ridges, the weak crystallization front may prohibit formation of an efficient barrier and lead to the possibility that some fraction of melt may be incorporated into the lithospheric mantle, allowing refertilization. The protracted crystallization history and potential absence of an effective permeability barrier may explain the dearth of volcanism at ultraslow ridges and calls for a revision of lateral melt focusing scenarios at ultraslow spreading rates.

International audience; Normal and transform faults originally formed at a spreading-centre can be reactivated in diffuse plate boundary zones and in areas of lithospheric flexure such as at peripheral bulges to subduction zones. Using new seismic reflection profiles and modeling, we investigate how the original oceanic fabric is reactivated in the simple case of fault perpendicular compression. In the Central Indian Basin, well-oriented normal paleofaults were reactivated with reverse motion at the very onset of deformation (9 Ma) but only a small subset remained active past $7 Ma, suggesting that most of the densely spaced small-offset faults were abandoned while deformation localized onto fewer faults with larger spacing. We find a similar evolution using a 2D finite element code of lithospheric shortening using a pseudoplastic rheology. Weak zones, 3 km-spaced and 30 – 40% weaker than the surrounding material, are introduced to simulate the fabric formed at the ridge axis. We show that reactivation and selective abandonment require strain weakening followed by strain-rate weakening once a maturation threshold is reached. A maturation fault slip of less than 50 m is needed to produce a fault network similar to that in the Central Indian Ocean.

[1] Mid-ocean ridge morphology correlates strongly with spreading rate. As the spreading rate decreases, conductive cooling becomes more important in controlling ridge thermal structure and the axial lithosphere thickens. At ultraslow spreading rates, the ridge axis becomes sufficiently cold that peridotite blocks are emplaced directly at the seafloor and volcanism is limited to localized volcanic centers widely spaced along the ridge axis. Some slow-spreading ridges adopt an ultraslow morphology when their axis is oblique to the spreading direction. We present an analytical solution for mantle flow beneath an oblique ridge and demonstrate that the thermal structure and crustal thickness are controlled by the effective spreading rate (product of the plate separation velocity and the cosine of obliquity). A global compilation of oblique ridges reveals that ultraslow morphology corresponds to effective half rates less than 6.5 mm/yr, resulting in lithosphere that is thicker than ∼30 km. We conclude that the transition from slow to ultraslow spreading is not related to a change of melt productivity but rather in the efficiency of vertical melt extraction.

[1] A fundamental understanding of the effect of stress on permeability evolution is important for many fault mechanics and reservoir engineering problems. Recent laboratory measurements demonstrate that in the cataclastic flow regime, the stress-induced anisotropic reduction of permeability in porous rocks can be separated into 3 different stages. In the elastic regime (stage I), permeability and porosity reduction are solely controlled by the effective mean stress, with negligible permeability anisotropy. Stage II starts at the onset of shear-enhanced compaction, when a critical yield stress is attained. In stage II, the deviatoric stress exerts primary control over permeability and porosity evolution. The increase in deviatoric stress results in drastic permeability and porosity reduction and considerable permeability anisotropy. The transition from stage II to stage III takes place progressively during the development of pervasive cataclastic flow. In stage III, permeability and porosity reduction becomes gradual again, and permeability anisotropy diminishes. Microstructural observations on deformed samples using laser confocal microscopy reveal that stress-induced microcracking and pore collapse are the primary forms of damage during cataclastic flow. A probabilistic damage model is formulated to characterize the effects of stress on permeability and its anisotropy. In our model, the effects of both effective mean stress and differential stress on permeability evolution are calculated. By introducing stress sensitivity coefficients, we propose a first-order description of the dependence of permeability evolution on different loading paths. Built upon the micromechanisms of deformation in porous rocks, this unified model provides new insight into the coupling of stress and permeability.

Gravity and bathymetry data from transform fault systems associated with mid-ocean ridges that are spreading at different rates indicates that crustal thickening may occur under transform faults associated with intermediate- and fast-spreading ridges. If confirmed, this observation would require a change in our understanding of how magma accretion occurs along rapidly spreading mid-ocean ridges. Mid-ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge. Slow-spreading mid-ocean-ridge segments exhibit significant crustal thinning towards transform and non-transform offsets1,2,3,4,5,6,7,8,9,10,11,12, which is thought to arise from a three-dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments1,2,4,5,6,7,9,10,12. In contrast, fast-spreading mid-ocean ridges are characterized by smaller, segment-scale variations in crustal thickness, which reflect more uniform mantle upwelling beneath the ridge axis13,14,15. Here we present a systematic study of the residual mantle Bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate. Previous studies have shown that slow-slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments1,2,4,6, but our analysis reveals that intermediate and fast-slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments. This finding indicates that there is a mass deficit at intermediate- and fast-slipping transform faults, which could reflect increased rock porosity, serpentinization of mantle peridotite, and/or crustal thickening. The most negative anomalies correspond to topographic highs flanking the transform faults, rather than to transform troughs (where deformation is probably focused and porosity and alteration are expected to be greatest), indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed. This finding in turn suggests that three-dimensional magma accretion may occur near intermediate- and fast-slipping transform faults.

[1] We analyzed in detail the geology of the median valley floor of the Mariana Basin slow-spreading ridge using sea surface geophysical data and a high-resolution deep-tow side-scan sonar survey over one spreading segment. Analysis of surface magnetic data indicates highly asymmetric accretion, with the half-spreading rate on the western side of the basin being two to three times larger than on the eastern side. Surface magnetic and reflectivity data together suggest that asymmetric spreading is accomplished through eastward ridge jumps of ∼10 km of amplitude. Deep-tow backscatter data indicate along-axis variations of the volcanic processes with the emplacement of smooth and hummocky flows at the segment center and end, respectively. This variation likely relates to changes in the effusion rate due to the deepening or even disappearance of the magma chamber toward the segment end. Concerning tectonic processes, we find a power law distribution of the fractures, with an exponent of 1.74. This suggests that within the inner valley floor, fracture growth prevails over fracture nucleation and coalescence and that fractures accommodate less than 8% of the strain. According to our calculation based on a ratio of 0.02 to 0.03 between the vertical displacement and the length of faults, the amount of tectonic strain accommodated in the inner valley floor would consistently be ∼1.1–3.4%. Data also show two distinct sets of fractures. One trend is parallel to the rift direction at the segment center (∼N160°E) and perpendicular to the plate separation direction. Another set trends ∼17° oblique to this direction (∼N175°E) and is located over the eastern part of the valley, in the vicinity of a major bounding fault also trending ∼N175°E, that is, obliquely to the direction of plate motion. We modeled the stress field near a major fault that is oblique to the regional stress field associated with plate separation using a three-dimensional boundary element approach. We found that the orientation of the predicted fissuring near the oblique fault is locally rotated by ∼15° due to a flexure of the bending plate close to this fault.

[1] Postseismic deformation is well documented in geodetic data collected in the aftermath of large earthquakes. In the postseismic time interval, GPS is most sensitive to creep in the lower crust or upper mantle activated by earthquake-generated stress perturbations. In these regions, deformation may be localized on an aseismic frictional surface or on a ductile shear zone. These two hypotheses imply specific rheologies and therefore time dependence of postseismic creep. Hence postseismic creep constitutes a potential probe into the rheology of aseismic regions of the lithosphere. I present a simple shear zone model of postseismic creep in which the rheology of the creeping element can be varied. In the absence of tectonic loading during the postseismic time interval, the displacement history of the shear zone obeying a power law rheology with stress exponent n follows an analytical relaxation curve parameterized by 1/n. For a frictional surface, postseismic creep follows the same relaxation law in the limit 1/n → 0. A rough estimate of the apparent stress exponent can be obtained from continuous GPS records. Application to data collected after the 1994 Sanriku earthquake yields 1/n ∼ 0.1, which is consistent with dislocation creep mechanisms. However, the records of two other subduction zone events, the 2001 Peru event and the 1997 Kronotski earthquake, and a continental strike-slip earthquake, the 1999 Izmit earthquake, require negative 1/n. Rather than characterizing the shear zone rheology, these negative exponents indicate that reloading of the shear zone by tectonic forces is important. Numerical simulations of postseismic deformation with nonnegligible reloading produce curves that are well fit by the generalized relaxation law with 1/n < 0, although the actual stress exponent of the rheology is positive. While this prevents rheology from being tightly constrained by the studied GPS records, it indicates that reloading is important in the postseismic time interval. In other words, the stress perturbation induced by an earthquake is comparable to the stress supported by ductile shear zones in the interseismic period.

(1) Tectonic deformation in the Central Indian Basin (CIB) is organized at two spatial scales: long-wavelength (� 200 km) undulations of the basement and regularly spaced faults. The fault spacing of order 7-11 km is too short to be explained by lithospheric buckling. We show that the localization instability derived by Montesi and Zuber (2003) provides an explanation for the fault spacing in the CIB. Localization describes how deformation focuses on narrow zones analogous to faults. The localization instability predicts that localized shear zones form a regular pattern with a characteristic spacing as they develop. The theoretical fault spacing is proportional to the depth to which localization occurs. It also depends on the strength profile and on the effective stress exponent, ne, which is a measure of localization efficiency in the brittle crust and upper mantle. The fault spacing in the CIB can be matched by ne �� 300 if the faults reach the depth of the brittle- ductile transition (BDT) around 40 km or ne �� 100 if the faults do not penetrate below 10 km. These values of ne are compatible with laboratory data on frictional velocity weakening. Many faults in the CIB were formed during seafloor spreading. The preexisting faults near target locations separated by the wavelength of the localization instability were preferentially reactivated during the current episode of compressive tectonics. The long- wavelength undulations may result from the interaction between buckling and localization. INDEX TERMS: 8010 Structural Geology: Fractures and faults; 8020 Structural Geology: Mechanics; 8159 Tectonophysics: Evolution of the Earth: Rheology—crust and lithosphere; KEYWORDS: faults, Central Indian Basin, fault spacing, folds, strength envelope, diffuse plate boundary

[1] Large-scale tectonic structures such as orogens and rifts commonly display regularly spaced faults and/or localized shear zones. To understand how fault sets organize with a characteristic spacing, we present a semianalytical instability analysis of an idealized lithosphere composed of a brittle layer over a ductile half-space undergoing horizontal shortening or extension. The rheology of the layer is characterized by an effective stress exponent, ne. The layer is pseudoplastic if 1/ne = 0 and forms localized structures if 1/ne < 0. Two instabilities grow simultaneously in this model: the “buckling/necking instability” that produces broad undulations of the brittle layer as a whole, and the “localization instability” that produces a spatially periodic pattern of faulting. The latter appears only if the material in the brittle layer weakens in response to a local perturbation of strain rate, as indicated by 1/ne < 0. Fault spacing scales with the thickness of the brittle layer and depends on the efficiency of localization. Localization is more efficient for more negative 1/ne, leading to more widely spaced faults. The fault spacing is related to the wavelength at which different deformation modes within the layer enter a resonance that exists only if 1/ne < 0. Depth-dependent viscosity and the model density offset the instability wavelengths by an amount aL that we determine empirically. The wave number of the localization instability, is kjL = π(j + aL)(−1/ne)−1/2/H, with H the thickness of the brittle layer, j an integer, and 1/4 < aL < 1/2 if the strength of the layer increases with depth and the strength of the substrate decreases with depth.

[1] Localized regions of deformation such as faults and shear zones are ubiquitous in the Earth's lithosphere. However, we lack a simple unified framework of localization that is independent of the mechanism or scale of localization. We address this issue by introducing the effective stress exponent, ne, a parameter that describes how a material responds to a local perturbation of an internal variable being tested for localization. The value of ne is based on micromechanics. A localizing regime has a negative ne, indicating a weakening behavior, and localization is stronger for more negative 1/ne. We present expressions for the effective stress exponent associated with several mechanisms that trigger localization at large scale: brittle failure with loss of cohesion, elastoplasticity, rate- and state-dependent friction, shear heating, and grain-size feedback in ductile rocks. In most cases, localization does not arise solely from the relation between stress and deformation but instead requires a positive feedback between the rheology and internal variables. Brittle mechanisms (failure and friction) are generally described by ne of the order of −100. Shear heating requires an already localized forcing, which could be provided by a brittle fault at shallower levels of the lithosphere. Grain size reduction, combined with a transition from dislocation to diffusion creep, leads to localization only if the grain size departs significantly from its equilibrium value, because either large-scale flow moves rocks through different thermodynamic environments or new grains are nucleated. When shear heating or grain-size feedback produce localization, 1/ne can be extremely negative and can control lithospheric-scale localization.

In early January 2004, one of us attended a workshop on “science priorities and educational opportunities that can be addressed using ocean observatories.” The attendees constituted a broad group—men and women, scientists, engineers, educators, representatives from the private and public sector—but lacked diversity in at least one important aspect: age. A well-known marine geophysicist (with a published record stretching over 30 years) came to me at the ice-breaker party and said (and I paraphrase):“I'm glad you're here:you're young, you might actually see this project flourish before you retire. There're not enough young people here.” At some point or another, every young scientist may have a similar experience.

The Galapagos triple junction is a ridge-ridge-ridge triple junction where the Cocos, Nazca, and Pacifi c plates meet around the Galapagos microplate. Directly north of the large scarps of the Cocos-Nazca Rift, a 250-km-long and 50-km-wide band of northwest-southeast‐trending cracks with volcanics at their western ends crosscuts and blankets the north-south‐trending abyssal hills of the East Pacifi c Rise. It appears that the roughly northeast-southwest extension of East Pacifi c Rise‐generated seafl oor has been accommodated by a succession of minor rifts that, during at least the past 4 m.y., had their triple junctions with the East Pacifi c Rise at distances of 50‐100 km north of the tip of the propagating Cocos-Nazca Rift. We propose that the rift locations are controlled by stresses associated with the dominant Cocos-Nazca Rift, and scaled by the distance of its tip to the East Pacifi c Rise. We speculate that similar ephemeral rifts occurred south of the Cocos-Nazca Rift and were instrumental in the origin of the rotating Galapagos microplate ca. 1.5 Ma.

Young researchers face a wide choice of scientific approaches and directions that may shape their careers. The Earth sciences, in particular, offer a broad range of topics to study and techniques to use that exceeds what any current scientist was exposed to at the schools they attended. At early stages of their careers, researchers need to gain confidence in their expertise, and they often can benefit by expanding their scientific horizons and forging new collaborations

The first Meeting of Young Researchers in the Earth Sciences (MYRES-I), held in August of 2004, focused on ?Heat, helium, hotspots, and whole mantle convection.? Biennial meetings, with MYRES-I as the first, are one of the ways the MYRES initiative is building an ?international, interdisciplinary, open and unbiased community of colleagues who interact regularly to informally exchange ideas, data, and tools, and formulate new collaborative research projects? (see Young Solid Earth Researchers of the World Unite! published in Eos, 85(16), 160, 2004). This article reports on our first workshop, discusses what is happening in the community, and calls for proposals to keep MYRES funded. A New Meeting Concept. The MYRES meetings are organized by, and for, junior members of the solid Earth research community. In 2004, funding from the National Science Foundation (NSF), the European Science Foundation (ESF) and The Scripps Institution of Oceanography enabled the initiative to nearly fully fund this meeting with a diverse and international crowd of nearly 100 participants selected from an oversubscribed pool.

A good understanding of the rheology and strength of the whole crust is needed to obtain a physics-based earthquake prediction models. However, geodynamics-based and laboratory-based strength estimates disagree. Geodynamics tend to indicate that the actual strength of the plastic crust is less than deduced from laboratory experiments. Here, I evaluate lower crust strength from observations of transient postseismic deformation. Fault motion during an earthquake produces only a small stress perturbation, but that perturbation is sufficient to significantly affect the deformation rate of the aseismic levels of the crust, as observed by space geodesy. Even considering the non-linearity of plastic flow in geological materials, one cannot escape the conclusion that the pre-earthquake stress on the region where transient postseismic deformation occurs is not more than an order of magnitude larger than earthquake-induced stress perturbations. Using a simple shear zone model and assuming wet quartzite rheology, I show that such stress levels are not compatible with a km-scale shear zone, in spite of the geological evidence for localized deformation in the plastic crust. This implies that in plastic shear zones, rock strength is reduced. Possible explanations for the strength reduction include structural effects such as reduced grain size and/or a localized thermal anomaly associated with the shear zone.

Geochemical observations of ocean island and mid‐ocean ridge basalts suggest that abundances of heat‐producing elements (HPEs: U, Th, and K) vary within the mantle. Combined with bulk silicate Earth models and constraints on the Earth's heat budget, these observations suggest the presence of a more enriched (potentially deep and undepleted) reservoir in the mantle. Such a reservoir may be related to seismically observed deep mantle structures known as large low shear velocity provinces (LLSVPs). LLSVPs might represent thermochemical piles of an intrinsically denser composition, and many studies have shown such piles to remain stable over hundreds of Myr or longer. However, few studies have examined if thermochemical piles can remain stable if they are enriched in HPEs, a necessary condition for them to constitute an enriched HPE reservoir. We conduct a suite of mantle convection simulations to examine the effect of HPE enrichment up to 25× the ambient mantle on pile stability. Model results are evaluated against present‐day pile morphology and tested for resulting seismic signatures using self‐consistent potential pile compositions. We find that stable piles can form from an initial basal layer of dense material even if the layer is enriched in HPEs, depending on the density of the layer and degree of HPE enrichment, with denser basal layers requiring increased HPE enrichment to form pile‐like morphology instead of a stable layer. Thermochemical piles or LLSVPs may therefore constitute an enriched reservoir in the deep mantle.